1. Field of the Invention
The present invention relates to an improvement in low pressure metal vapor discharge lamps and, more particularly, to a single base fluorescent lamp the size of which is reduced by the use of a double-tube structure for the discharging vessel.
2. Description of the Prior Art
A conventional low pressure metal vapor discharge lamp, which is well represented by an ordinary lighting fluorescent lamp (low pressure mercury vapor discharge lamp), has an elongated glass tube provided at both ends with electrodes and accommodating a rare gas of several Torrs pressure and a small amount of a metal such as mercury. This type of lamp has a large length for its lighting power. For instance, the length of a straight-tube type fluorescent lamp provided at both sides with bases well reaches 120 cm for an electric input of 40 W.
This double base type fluorescent lamp having a large length is, in some cases, extremely inconvenient. Rather, this type of lamp is not recommended for some specific uses. This gives rise to a demand for a smaller fluorescent lamp having a reduced tube length while preserving good brightness.
To comply with this demand, Japanese Patent Publication No. 35796/1974 discloses a novel fluorescent lamp having only one base. The proposed fluorescent lamp has a double-tube structure consisting of a fully closed outer glass bulb and an inner glass tube disposed within the outer glass bulb, the inner glass tube being open at one end but closed at the other end. One (cathode) of the electrodes is disposed within the inner glass tube, while the other (anode) electrode is disposed outside the inner glass tube. According to this arrangement, the discharge path formed between two electrodes makes a turn at the opening end of the inner glass tube, so that a sufficiently large length of the discharge path can be obtained with a relatively small length of the lamp.
Further, the luminous efficiency is expectedly improved because the area of the glass tube surrounding the discharging space is increased to provide a larger area of the surface to which the luminous paint phosphor material is applied.
This known double-tube type of discharge lamp, however, has a problem in that it is difficult to uniformly distribute the discharge plasma over the entire discharging space between the inner glass tube and the outer glass bulb.
Namely, the discharge plasma outside the inner glass tube is concentrated locally to the region which exhibits the smallest resistance to the discharge current, and is not spread uniformly over the entire discharging space. This local concentration of the discharge plasma can never be avoided even by the use of a ring-shaped anode disposed around the inner glass tube. In such a lamp, the luminous intensity is specifically high only at the region to which the plasma is locally concentrated, while only a low luminous intensity is obtained at portions of the lamp where the plasma is not distributed. Thus, it is difficult to obtain a uniform luminous intensity distribution over the entire lamp body.
In addition, in the double-tube type lamp of the kind described, the discharge plasma which is locally concentrated to a portion of the discharging space changes its position irregularly so as to cause the so-called "flickering phenomenon".
In order to overcome the problem concerning the local concentration of the plasma in the known double-tube type fluorescent lamp, the specification of U.S. Pat. No. 3,609,436 as well as the Journal of the Illuminating Engineering Society (Vol. 2, No. 2, October 1972, pages 3-7), proposes an improved lamp in which a plurality of anodes are disposed around the inner glass tube. These anodes are switched successively so as to forcibly rotate the locally concentrating plasma at a high speed around the inner glass tube to thereby achieve a uniform luminous intensity over the entirety of the lamp.
This improved lamp, however, requires a complicated and expensive transistor switching circuit for a high-speed switching of the voltage over the successive anodes and, therefore, is not practical from both technological and economical points of view.